R-glycosides are known to be important intermediates for the production of fine chemicals, including sugar-based surfactants. Ordinarily, R-glycosides are prepared by Fischer glycosidation of an R-alcohol with a sugar, which involves the acid catalyzed formation of a glycoside bond between the acetal or ketal carbon of the sugar and the hydroxyl group of the alcohol. The most common sugar is glucose. R-glycosides can also be prepared by acid catalyzed Fischer glycosidation of glucose residues in a polysaccharide such as starch or cellulose with an alcohol, which results in cleavage of the glycosidic bonds in the polysaccharide via substitution of the alcohol moieties forming the free glucosides. Strong acids, elevated temperatures, and elevated pressures are typically needed. A mechanism compatible with milder conditions and utilizing a less expensive starting material, especially a starting material with otherwise limited applications, would be economically advantageous, especially on an industrial scale.
Cellulose is a primary component of plant matter, is non-nutritive, and is not widely utilized outside of the paper and textile industries. Cellulose can be converted to glucose through acid or enzymatic hydrolysis, however, hydrolysis is difficult due to the robust crystalline structure of cellulose. Known acid hydrolysis methods typically require concentrated sulfuric acid to achieve good yields of glucose. Unfortunately glucose in the presence of concentrated sulfuric acid can degrade to form hydroxymethylfurfural (“HMF”) which in turn can further polymerize into a tarry substance known as humins. The formation of HMF and tarry humins negatively impacts the yield of glucose and requires additional separation steps. Enzymatic hydrolysis methods known in the art are also impractical for industrial scale conversion of cellulose to glucose due to low reaction rates and expense and enzymes do no hydrolyze cellulose that has been chemically modified.
Recently, Deng et al. reported the direct conversion of cellulose and methanol into methyl glucosides in the presence of an acid catalyst. Deng et al., Acid-catalysed Direct Transformation of Cellulose into Methyl Glucosides in Methanol at Moderate Temperatures, 46 Chem. Comm. 2668-70 (2010). Various dilute mineral and organic acids were tested, with sulfuric acid providing the best yield of methyl glucosides at 48%. Keggin-type heteropolyacids were also tested, with H3PW12O40 yielding 53% methyl glucosides. However, the conversion of cellulose in ethanol in the presence of H3PW12O40 resulted in a decreased yield of 42% ethyl glucosides. Solid acids were tested, with various forms of carbon bearing SO3H groups giving the best yield of methyl glucosides at 61%.
More recently, Dora et al. reported the catalytic conversion of cellulose into methyl glucosides over sulfonated carbon based catalysts. Dora et al., Effective Catalytic Conversion of Cellulose into High Yields of Methyl Glucosides over Sulfonated Carbon Based Catalyst, 120 Bioresource Technology 318-21 (2012). Carbon based catalysts containing SO3H groups were synthesized and evaluated for the conversion of cellulose in methanol. Specifically, microcrystalline cellulose was reacted with methanol and the sulfonated carbon catalyst (50% by weight of the microcrystalline cellulose) at temperatures from 175° C. to 275° C. A maximum 92% yield of methyl glucosides was obtained at a reaction time of 15 minutes at 275° C.
Turning to sugar alcohols, here are currently no known processes for producing sugar alcohols (i.e. hexitols or pentitols such as sorbitol and xylitol) from alkyl glycosides by hydrogenation. Typically sugar alcohols are produced by heating unmodified sugars at elevated pressure in the presence of a hydrogenation catalyst.
Recently, Fukuoka et al. reported that sugar alcohols could be prepared from cellulose using supported platinum or ruthenium catalysts, which showed high activity for the conversion of cellulose into sugar alcohols with the choice of support material being important. Fukuoka et al., Catalytic Conversion of Cellulose into Sugar Alcohols, 118 Agnew. Chem. 5285-87 (2006). The mechanism involves the hydrolysis of cellulose to glucose followed by the reduction of glucose to sorbitol and mannitol. However the yields were at best around 30% conversion to sugar alcohols, and the reactions took place at an elevated pressure of 5 MPa.
More recently, Verendel et al. reviewed one-pot conversions of polysaccharides into small organic molecules under a variety of conditions. Verendel et al., Catalytic One-Pot Production of Small Organics from Polysaccharides, 11 Synthesis 1649-77 (2011). Hydrolysis-by-hydrogenation of cellulose under acidic conditions and elevated pressure was disclosed as yielding up to 90% sorbitol, although these processes were categorized as “by no means simple.” The direct hydrolysis-hydrogenation of starch, inulin, and polysaccharide hydrolysates to sugar alcohols by supported metals under hydrogen without the addition of soluble acids was also disclosed. Ruthenium or platinum deposited on aluminas, a variety of metals supported on activated carbon, and zeolites were reported as suitable catalysts for cellulose degradation. The effect of transition-metal nanoclusters on the degradation of cellobiose was also disclosed, with acidic conditions yielding sorbitol. A different study looked at the conversion of cellulose with varying crystallinity into polyols over supported ruthenium catalysts, with ruthenium supported on carbon nanotubes giving the best yield of 73% hexitols.
There remains a need for cost-effective methods of producing sugar alcohols with high selectivity and through alternate pathways.
On yet another subject, the molecule 1,2,5,6-hexanetetrol (“HTO”) is a useful intermediate in the formation of higher value chemicals. HTO and other polyols having fewer oxygen atoms than carbon atoms may be considered a “reduced polyols.” Corma et al. discloses generally that higher molecular weight polyols containing at least four carbon atoms can be used to manufacture polyesters, alkyd resins, and polyurethanes. Corma et al., Chemical Routes for the Transformation of Biomass into Chemicals, 107 Chem. Rev. 2443 (2007).
Sorbitol hydrogenolysis is known to produce HTO, although typically the reaction conditions are harsh and non-economical. U.S. Pat. No. 4,820,880 discloses the production of HTO involving heating a solution of a hexitol in an organic solvent with hydrogen at an elevated temperature and pressure in the presence of a copper chromite catalyst. Exemplary starting hexitols include sorbitol and mannitol. Water was found to adversely affect the reaction speed requiring the reaction to be performed in the absence of water and instead using ethylene glycol monomethyl ether or ethylene glycol monoethyl ether as the sole solvent, which puts a solubility limit on the amount sorbitol that can be reacted. Under such conditions the maximum concentration of sorbitol that was shown to be useful was 9.4% wt/wt in ethylene glycol monomethyl ether, which provided a molar yield of about 28% HTO. In a similar reaction where the sorbitol concentration was reduced to about 2% wt/wt in glycol monomethyl ether, the molar yield of HTO was 38% however the low concentration of reactants makes such a process uneconomical. More recently, U.S. Pat. No. 6,841,085 discloses methods for the hydrogenolysis of 6-carbon sugar alcohols, including sorbitol, involving reacting the starting material with hydrogen at a temperature of at least 120° C. in the presence of a rhenium-containing multi-metallic solid catalyst. Nickel and ruthenium catalysts were disclosed as traditional catalysts for sorbitol hydrogenolysis, however these catalyst predominantly produced lower level polyols such as glycerol and propylene glycol and were not shown to detectably produce HTO or hexanetriols.
There remains a need for improved cost-effective catalyst for producing HTO from sugar alcohols and a need for alternative substrates other than sugar alcohols.
On another background subject, the molecule 2,5 bis(hydroxymethyl)tetrahydrofuran (“2,5-HMTHF”) is typically prepared by the catalyzed reduction of HMF. This is impractical due to the expense of HMF, harsh reaction conditions, and poor yields. For example, U.S. Pat. No. 4,820,880 discloses the conversion of HTO to 2,5-HMTHF in ethylene glycol monomethyl ether with hydrogen at a pressure of at least 50 atmospheres, in the presence of a copper chromite catalyst, at a temperature in the range of 180° C. to 230° C.
Overall, there is a need in the art to devise economical methods for converting cellulose to alkyl glycosides, for converting alkyl glycosides to sugar alcohols, for converting sugar alcohols to HTO and other reduced polyols, and for making useful derivatives of such reduced polyols such as 2,5-HMTHF.